1. Field of the Invention
The present invention relates to an image reading position error measuring device which measures an image reading error from bit-map formation image data obtained from reading images through a reading device.
2. Description of the Related Art
As a first example of the related art, Shunsuke Hattori et al. disclose A Development of Image Scanner of High Resolution in the Japan Society of Mechanical Engineers, 71st ordinary general meeting, lecture meeting, lecture paper collection (IV) [Mar. 29-31, 1994, Tokyo]. Therein, an interpolation operation is performed on image data which is obtained as a result of reading a test chart having even-pitch lines arranged therein in a sub-scan direction. That image data is image data which is discrete in those sub-scan direction line intervals. From the interpolation operation result, the central positions of black lines and white lines of the even-pitch lines are obtained. Then, differences between the central positions and the reference pitch of the test chart are read. Thereby, image data reading position errors due to apparatus vibration or the like is detected.
As a second example of the related art, Japanese Laid-Open Patent Application No.6-297758 discloses A Scan-line Pitch Measuring Method. Therein, a pattern of a hard copy having even-pitch pattern data written therein is read. Thereby, unevenness in the pitch of scan lines which are used in a hard-copy apparatus is measured.
An optical linear scale (as a third example of the related art), for example, is disclosed in Base and Application of Servo Sensor written by Koujiro Ohshima and Yuji Akiyama, published by the Ohm company on Feb. 20, 1988. The disclosure in this publication will now be described with reference to FIGS. 1, 2A, 2B, 2C and 3.
Here, as an example of the linear scale, a position scale is used. The linear scale in the example, as shown in FIG. 1, includes a glass scale 83, a light source 84 and photo diodes 85. The glass scale 83 includes one set of a main scale 81 and an index scale 82, each scale having completely even-pitch light and shade stripe series. The LED light source 84 lights the scale 83. The photodiodes 85 detect light which has passed through the scale 83. Ordinarily, the index scale 82 is fixed and the main scale moves. As the main scale moves, the outputs of the photodiodes change.
As shown at the left of FIG. 2A, when the transmitting portions of the two glasses are aligned, the light output having passed through the glasses 81 and 82 is maximum. As shown at the right of FIG. 2A, when the transmitting portions of one glass are aligned with the chrome-deposited non-transmitting portions of the other glass, the light output having passed through the glasses 81 and 82 is zero. Accordingly, the light output waveform is ideally that shown in FIG. 2B. However, actually, because the stripe series light and shade pitch is as small as 8 .mu.m, due to influence of light diffraction and reflection on the chrome-deposited surfaces, an approximate sine wave is output, as shown in FIG. 2C. The span between each of the adjacent peaks of the output waveform corresponds to each pitch of the scale. Accordingly, by counting the peaks, an amount of movement of the main scale 81 can be known. This is the basic principle of the position scale. Actually, various processes are performed using the four photodiodes A, B, A, B.
The four stripe series of the index scale 82 are aligned with the four photodiodes A, B, A, B, respectively. The phase relationship among the four stripe series of the index scale 82 are 0.degree., 90.degree., 180.degree. and 270.degree.. The outputs of the four photodiodes A and A are combined and B and B are combined and differential operations are performed on each combination of the outputs. Thus, even if the scale 82 gets dirty and/or the light intensity of the light source 84 changes, an accurate result can be obtained. Thus, reliability is improved. The obtained signals will be referred to as `A signal` and `B signal`. The signals obtained as a result of electrically inverting these signals are referred to as `A signal` and `B signal`. Using these signals, a process of reading a smaller size is performed.
The direction in which the main scale 81 moves can be determined as a result of knowing which of the A signal and B signal leads in phase, as shown in FIG. 3. In order to read a size smaller than each pitch of the scale 83, using only the A signal, by taking a position at which the signal crosses the reference level from the lower side and a position at which the signal crosses the reference level from the higher side, the amount of movement of the scale can be read every 4 .mu.m. In order to read more finely, it is necessary to produce a 45.degree. phase difference signal using the A signals and B signal, and also, produce a 135.degree. phase difference signal using the B signal and A signal.
In the above-described first example of the related art, due to possible spatial differences between the edges of the even-pitch lines and sampling positions, `Moire` effect may occur wherein a difference occurs between data which has been obtained as a result of reading the same pattern. Due to the Moire effect, thus-obtained read data may not be data which indicates positions corresponding to the edges of the pattern. Thereby, accuracy in measuring image reading position errors may be degraded. Such an adverse effect is very noticeable when the even-pitch line pattern is so fine as to approximate the resolution of the reading apparatus. As a result, the measuring of image reading position errors may not be performed. Thus, using this method, it is not possible to measure, with a high accuracy, an image reading position error of a pattern which is so fine as to approximate or to be more than the resolution of the reading apparatus.
Further, because an even-pitch line pattern is used, even if the Moire effect is ignored, in a case where the pitch of the pattern is fine for measuring an image reading position error of a high-frequency component, due to the limitation of the MTF (Modulation Transfer Ratio) of the image formation system, a difference in a signal indicating image tone is disadvantageously reduced. Thus the measuring accuracy is degraded.
It is considered that, in the case where the pitch of the pattern is finer, the measuring frequency band is widened to a higher frequency. Thereby, it is not possible to provide a high measuring accuracy. Therefore, in order to solve this problem, the sampled data is made to undergo an interpolation operation. In order to improve the effect of the interpolation operation, it is necessary to increase an amount of surrounding data to be processed. As a result, a longer time is required for the operation. Further, the interpolation operation inherently may not provide true data, and thus the measuring accuracy may be degraded. Further, in the first example, image data to be used is obtained as a result of a specific light-reception element of the light-to-electricity converting device being used to scan the pattern in the sub-scan direction. The light-reception element itself may provide noise which may degrade the measuring accuracy.
In the above-described method of measuring in the second example of the related art, the light-to-electricity converting device is used to read the pattern and thus-obtained data is used. In this method, at this time, reading or scanning unevenness when reading or scanning a hard copy is not considered in measuring pitch unevenness in the pattern of the hard copy. Further, this method also has a `Moire` effect problem similar to that which occurs in the above-described first example of the related art.
In the above-described method of the third example of the related art, in the above-described linear scale, light emitted by the light source (LED) 84 becomes parallel light through a collimate lens 87. Then, light passing through the main scale 81 and index scale 82 is detected by the photodiodes. Accordingly, it is necessary to prepare a finely divided highly-accurate main scale 81 and index scale 82 and an accurate collimating lens is necessary. As a result, costs increase.
Therefore, the present applicant proposed solutions of the above-described problems in Japanese Patent Application Nos.7-256481 and 7-311015, and U.S. patent application Ser. No. 08/698,854.
However, the prior applications consider a case where equal size image reading (same-size image reading) is performed and do not consider a case where image size change reading is performed.
A case of performing image size change (image magnification or image size reduction of the read image) processing will now be described. Ordinarily, image size change processing is performed in an image reading apparatus, with regard to a sub scan direction, an having a changed size image is produced as a result of changing the carriage scanning speed. With regard to a main scan direction, an image size change is performed in image data processing. Accordingly, when the position error measuring portion is provided at the stage subsequent to processing of the main scan direction image size change, the angle of the oblique line in the read image is 45.degree. when the angle of the oblique line is 45.degree.. However, in the read image of the oblique line, the line width in the main scan line direction changes. When the magnification is large, the line width is large. When the magnification is small, the line width is small. Therefore, it is necessary to change the size of the window according to the magnification. When changing the window size according to the magnification, the gravity measuring time is different, and an error may occur in the position error measurement.